Organic EL devices utilizing organic substances are much expected to be useful as inexpensive, large-sized full color display devices of solid state emission type and many developments have been made thereon. An organic EL device is generally constructed from a light emitting layer and a pair of opposite electrodes sandwiching the light emitting layer. When an electric field is applied between the electrodes, electrons are injected from a cathode and holes are injected from an anode. The injected electrons recombine with the injected holes in the light emitting layer to form excited states. When the excited states return to the ground state, the energy is released as light.
Known organic EL devices have a higher driving voltage, a lower luminance, and a lower emission efficiency than those of inorganic light emitting diodes. In addition, the properties thereof are drastically deteriorated to make the practical use difficult. Although the organic EL device has been gradually improved, a high luminance and a high luminous efficiency at lower voltage has been required.
To solve the above problems, a device containing a compound having a benzimidazole structure as the light emitting material is disclosed in Patent Document 1. It is described that this device shows a luminance of 200 cd/m2 at a voltage of 9 V. A compound having a benzimidazole ring and an anthracene residue is described in Patent Documents 2 and 3. However, a luminance and a luminous efficiency still higher than those of organic EL devices employing these compounds are required.    Patent Document 1: JP 10-092578A (U.S. Pat. No. 5,645,948)    Patent Document 2: JP 2002-38141A    Patent Document 3: WO 2004/080975 (US 2006/147747)